1. Field of the Invention
The invention relates to a signal generator using an IIR type digital filter, which is used as a re-generator of, for example, mobile communication system; and a method of generating, supplying, and stopping its output signal.
2. Related Background Art
According to a conventional signal generator using an IIR type digital filter, at the start of the operation, an impulse delta[n] (delta[n] are set to “all 0” except in the case where n=0) is inputted to the IIR type digital filter expressed by the following difference equation.y[n]=x[n]+A1*y[n−1]−A2*y[n−2]  (1)By transmitting output data y[n] to a multiplier (coefficient B0) provided out of a feedback loop of the IIR type digital filter, a desired output signal is obtained.
Poles of a transfer function of the equation (1) are set on a unit circle on a Z plane, not depending on each time when output signal is generated, supplied or stopped.
It is assumed hereinafter that generation of output signal denotes a stage (period of time) until the output signal becomes stable after signal generator started operating, that supply of output signal denotes a stage (period of time) after the output signal became stable, and that stop of output signal denotes a stage (period of time) until the output signal stops after it became stable. Moreover, the output signal denotes a signal obtained by converting the output data (digital signal) into an analog signal.
In the above prior art, if the signal generator is inputted with delta[n] at starting time of operation, for the signal generator to start operating; a value of the output data changes suddenly by transitional response which appears with generation of output signal. Moreover, if the output signal is stopped by switching the coefficient B0 of the multiplier to 0, a value of the output data changes suddenly by transitional response which appears with stop of output signal. Such a sudden change in output data is converted into an analog signal and its harmonics penetrate, as noises, peripheral circuits. Accordingly, a problem to be solved such that the noises sometimes cause inconvenience still remains.
In order to solve this problem, there is also a case where the coefficient B0 is gradually decreased to 0 and the output is stopped. In this case, a problem to be solved such that it is necessary to execute arithmetic operations of the coefficient B0 many times and a control method becomes complicated still remains. Accordingly, a technique for enabling a calculation or the like of the transfer function to be easily executed has also been released (for example, refer to a patent literature 1: JP-A-6-61790 (Abstract)).
What is to be solved by present invention is: a problem such that a value of the output data changes suddenly by transitional response which appears with generation of output signal, or by transitional response which appears with stop of output signal; and such a sudden change in output data is converted into an analog signal and its harmonics penetrate, as noises, peripheral circuits; causing inconvenience to peripheral circuits.